Tabular silver halide grains are grains possessing two parallel crystal faces with an aspect ratio of two or more. Said aspect ratio is defined as the ratio between the diameter of a circle having an equivalent surface area as one of these crystal faces, and the thickness, being the distance between the two major faces.
Tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al. described the preparation and growth of tabular silver bromoiodide grains in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966,p.66-72. Early patent literature includes U.S. Pat. Nos. 4,063,951; 4,067,739; 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described herein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of U.S. Patents filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described as e.g. U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425 and 4,425,426 and in Research Disclosure, Volume 225, January 1983, Item 22534.
Anisotropic growth characteristics for the said tabular grains are known to be due to the formation of parallel twin planes in the nucleation step of the precipitation and as a global result fairly heterogeneous emulsion crystal distributions are obtained: a common variability coefficient (defined as a ratio between average standard deviation on equivalent circular diameter and the said average equivalent circular diameter) of 0.30 to 0.60 is calculated, partly due to the presence of quite a large number of non-tabular grains having a sphere equivalent diameter of less than 0.3 .mu.m. Moreover differences in thickness growth are observed, said differences leading to unevenness as a consequence of observed differences in image tone.
Heterodispersity of grain morphology further leads to e.g. uncontrolled chemical and spectral sensitization, lower contrast and lower covering power, thereby losing typical advantages of the said grains as referred to hereinbefore.
Until now efforts in order to get more monodisperse tabular silver halide crystal distributions in emulsion preparation have been directed towards silver halide crystals rich in silver bromide as has e.g. been described in U.S. Pat. Nos. 4,797,354; 5,147,771; 5,147,772; 5,147,773; 5,171,659; 5,248,587; 5,204,235; 5,210,013; 5,215,879; 5,250,403; 5,252,442, 5,252,453; 5,254,453; 5,318,888; 5,439,787; 5,472,837; 5,482,826 and 5,484,697 and in Research Disclosure No. 391, p. 713-723 (1996).
Many attempts have been made in order to improve the degree of homogeneity of the size and shape of the crystals but the majority of them is related with tabular grains rich in silver bromide. So radiographic materials comprising emulsions having monodisperse tabular silver brom(oiod)ide crystals have e.g. been described in U.S. Pat. Nos. 5,252,442 and 5,508,158.
Combinations of intensifying screens provided with luminescent phosphors and light-sensitive silver halide photographic materials are conventionally used for medical diagnosis. By X-ray radiation the luminescent phosphors in the screen panel or panels are converting X-rays into visible radiation, thereby exposing the film material in contact with the said panel (for single-side coated materials as e.g. in mammography) or panels (for duplitized or double-side coated materials as e.g. in chest imaging). So in mammography e.g. the compressed breast is irradiated with soft X-rays emitted from an X-ray generating device and the modulated X-rays are detected with a radiographic X-ray conversion screen, also called intensifying, fluorescent screen or "phosphor screen". The X-ray conversion screen comprises a luminescent phosphor which converts the absorbed X-rays into visible light and the emitted visible light exposes a silver halide film that is brought into contact with said X-ray conversion screen. After film processing, comprising the steps of developing, fixing, rinsing and drying, a mammogram is obtained which can be read on a light box. No other field of medical radiology demands such a high level of image quality as mammography and the ability of the mammogram to portray relevant diagnostic information is highly determined by the image quality of the screen-film system. Image quality is manifested by a number of features in the image including sharpness, noise, contrast, silver image colour and skin line perceptibility. Conventional mammography films can roughly be classified in low and high contrast types according to the value of their average gradation as defined above. The low contrast type can be characterized by a relatively low average gradation ranging from 2.0 to 2.5 whereas the average gradation of the high contrast type may range higher than 3.0. Often, high contrast films are preferred because of the higher ability to detect tiny cancers deep in the glandular tissue of the breast. If the contrast is too high, however, it may preclude visualization of both thin (i.e. the skin line) and thick tissues (i.e. the inside of the breast) in the same image due to lack of exposure latitude. Therefore, some radiologists prefer low contrast mammography films. When the contrast is low, skin line perceptibility is excellent, but then the chance of missing possibly malignant breast lesions is high. Thus a balance has to be found between contrast and exposure latitude and an example of this approach has been described in U.S. Pat. No. 5,290,655. At first sight tabular grains would be judged not to be able to satisfy all those requirements, especially with respect to contrast.
Maintaining the said contrast and image definition related therewith at a constant level is becoming another requirement of facilities performing mammography. Accordingly, quality control tests are executed on a regular basis in order to monitor the consistency of the performance of the X-ray equipment, the image receptors and the film processor. In order to minimize the influence of varying film processing time, temperature, chemistry and replenishment, a preferred mammography film requires a stable speed and contrast with regard to these processing parameters. As in addition, there is a general trend in the field of radiology to shorten the film processing time and likewise in the field of mammography, being driven by intensified screening programs, the interest has focused on rapid access of mammograms. As a consequence, mammography films are preferred which comprise silver halide crystals that can be processed rapidly and consistently in a dry-to-dry processing cycle of 90 seconds or less and therefore, most mammography films today comprise good developable cubic silver halide crystals. As described in EP-A 0 712 036 such cubic crystals show a stable speed and contrast upon varying processing parameters, but said cubic grain emulsions however are characterized by a very high contrast, resulting in a poor skin line perceptibility. Especially in rapid processing applications it is very difficult to obtain the desired low fog, high speed and high covering power simultaneously. Replacing cubic grain emulsions by tabular grain emulsions, although being favourable with respect to high covering power obtainable at moderate coating amounts of silver halide as has been demonstrated e.g. in U.S. Pat. No. 4,414,304, shows the disadvantageous lower contrast than the contrast obtainable with cubic grains, besides the appearance of a brown colour hue of developed crystals and the residual colouration of the processed image, especially in short processing cycles, due to strong adsorption of huge amounts of spectral sensitizing dye(s) at the large specific surface area, characteristic for the said tabular grains.
Making use of a mixture of cubic and tabular grains or of a multilayer arrangement of cubic and/or tabular grains in order to provide a good image tone as in EP-A 0 874 275 and in EP-A 0 770 909 respectively is more complex and less interesting from the point of view of reproducibility of the production process. As tabular grain emulsions having a high aspect ratio are known to provide, over more conventional cubic, octahedral or spherical grains, a high covering power and, as a consequence thereof, a lower coating weight, manufacturing costs are reduced. Said lower coating weight is especially preferred if rapid processing applications, preferably accompanied by low replenishing amounts of developer and fixer, are required, which is nowadays an ever more returning demand. Attaining the desired contrast however, even with lower coating amounts is another problem that should be overcome.